Magnetic Field "Blocker"

[d22k]

I originally posted this in a different section, but i think its more appropriate here.

 

I was lying in bed last night when an interesting idea struck me, however, it revolves around 1 important thing; I need material capable of completely blocking a magnetic field.

 

Bismuth is something tht i have considered, as it is diamagnetic, albeit weakly, but any other suggestions would be greatly appreciated.

 

Also, is bismuth "complete" in its diamagnetism? i.e will it completely reverse the magnetic field, so even though the reverse field will be weak, there is nothing left of the previous polarity?

 

i hope i made that clear enough :s

 

heres a picture to demonstrate

[calbiterol]

Are you trying to get a one-sided magnet?

[DQW]

The picture you've drawn can not be achieved as proposed...and even something close to what you've drawn can be made only with a superconductor (SC). SCs are your perfectly diamagnetic materials. They have a susceptibility [imath] \chi _m = -1[/imath] making [imath] B_{in} = H + M = H + \chi _m H = H ( 1+ \chi _m) = 0 [/imath]

 

Bismuth, while being a relatively strong diamagnet (compared to other normal diamegnets) has a susceptibility of only about [imath] \chi _m = -10^{-4} [/imath], so it will allow 99.99% of the field right through it.

[swansont]

You can encase the magnet in mumetal and shield a large fraction of the field that way.

[d22k]

yes a 1 sided magnet is exactly what i want.

is it possible to make one with this mumetal? it would seem so, from a quick google.

 

any ideas on the pricing?

 

Edit: OMG! i cant afford this!!

 

http://www.goodfellow.com/scripts/web.wl?MGWLPN=MNT&PROG=SEARTOW&LAN=A&HEAD=ZZNIAAAD&SPAGE=NI03

[DQW]

You can encase the magnet in mumetal and shield a large fraction of the field that way.

This is the traditional method of shielding. However, it does not do the kind of "shielding" the OP is talking about.

 

Mumetal shields regions outside it by restricting a large fraction of the field to itself. It is an excellent soft ferromagnet with a susceptibility of around 10⁵ (large, positive susceptibility). What the OP wants, is a material that will "completely reverse the magnetic field" - a perfect diamagnet (large, negative susceptibility).

 

The geometry described in the picture, however, can be achieved (roughly) either way. With a SC, the field lines pass through air (more likely liquid nitrogen or its cold vapor); with mumetal, the field lines pass through the metal. The first is harder to implement (no room temperature solution), but the second restricts the field lines to the inside of a metal.

[d22k]

ahh, i dont think i mad emy intentions clear!

the field does not have to be reversed on the right hand side, just so long as it isnt there.

 

I mentioned diamagnetism because thats the only method i was aware of.

 

Would mumetal do this, or something like it?

[DQW]

If what you are interested in the the weak field in the region indicated, then yes, mumetal will do just that. But a side effect, is that this will also considerably weaken the field on the other side, though not as much as on the side that the mumetal covers.

[d22k]

interesting, it MAY work for the purposes i intend.

 

Are you aware of any cheaper ways to do the same thing. that works out to about $75 per square centimeter :s

[calbiterol]

yes a 1 sided magnet is exactly what i want.[/url]

 

A Halbach array is probably your best option.

 

It uses normal magnets, so it can be (more or less) as expensive or inexpensive as you want, according to your power needs, and it augments the field on one side, and pretty much cancels out the field on the other. The link is wikipedia's article on it, and the article also has some good links.

[d22k]

wow! thats absolutely perfect!

thanks alot, i will let you know how the project progresses.

[calbiterol]

Glad to help.

[DQW]

A Halbach array is probably your best option.

 

It uses normal magnets' date=' so it can be (more or less) as expensive or inexpensive as you want, according to your power needs, and it augments the field on one side, and pretty much cancels out the field on the other. The link is wikipedia's article on it, and the article also has some good links.[/quote']I may be completely off here, but it seems to me like you can only change the field along the z-direction (using a halbach array or some winding scheme with electromagnets). My z-axis points along the symmetry axis of the magnet, so I'm not sure how you get the field-variation that d22k wants along the x-direction.

 

I just might be missing something obvious, though.

[d22k]

well, im planning on using cube rare earth magnets, so asuming i built an array, what would the field look like in this pic?

 

(you will have to excuse the lack of scale, accuracy, and drawing skills )

[DQW]

If you showed us the directions of the magnetizations of each of your cubes, one might be able to make a rough guess of the field geometry.

[d22k]

well i simply dont know. I know NOTHING about magnets, so i cant give u any useful information =\

 

I guess im just gonna have to build it an see if it works =)

[calbiterol]

This page has a bunch of more technical information on Halbach arrays.

 

If you let us know your idea, we could tell you if it would work or not, and/or help you out with it. I understand not wanting to divulge your ideas, but it would be much easier for us to help you. Either way, good luck.

[d22k]

i plan to tell all, eventually

 

im not sure if its gonna work u see... but if it does work its gonna be freakin cool!

 

so i just need to save up some cash for the magnets and try it out.

 

If it doesnt work, it least i will have some cool magnets to play with

[calbiterol]

Incredibly fun. If it doesn't work, you should make a gauss rifle. Those are really fun.

[d22k]

tell me about it!

 

i was sketching up some plans just yesterday for a gauss "rifle" using 4 electro magnets, i have everything i need except 4 suitable pieces of iron =)

 

I was reluctant t call it a rifle, because it isnt... but it just sounds cooler than "gauss gun" doesnt it

[calbiterol]

Yep. I can't say it takes much to turn it into a rifle, though! Those things can pack such a punch... I'm really rather surprised that the military hasn't done something with the idea... You'd think, with the fact that you don't need any dangerous explosives. But then again, what is logical these days?

[d22k]

well i would be surprised if they havnt and/or arnt experimenting with gauss rifles.

 

thing is, for it to be effective as a weapon, its gonna need a big power source, and technology for a compact yet potent power source is still developing, i would think that would be the limiting factor.

 

they HAVE developed a gun that's uses a centrifuge to fore ball-bearings though. Apparently it can fire at huge velocities, fair accuracy and near silent, while at the same time creating a huge volume of fire.

 

it fires so rapidly it can deliver bullets to the target only 8mm apart!

 

wheee

 

EDIT: LINK - http://www.newscientist.com/article.ns?id=dn7365

[calbiterol]

Speed is often inversely proportional to precision (in shooting, this is known as a pattern). Being a taget shooter myself, 8mm apart is pretty bad, but I guess your link will have to divulge the information. As for an effective gauss rifle - they don't require any power to fire at all - gravity could do it, if the engineering was done correctly. Resetting the firing mechanism requires the power, and in this case, if the sear (the thing that catches bolt) is non-magnetic, you only have to have enough power to move a ball bearing. With magnetic levitation of bullets, extreme accuracy can be achieved. Needless to say, I've thought the system out.

 

 

[Edit: I'm reading the site now. So far, I'm not incredibly impressed by the technology - it's stealth possibilities are not that impressive, and overall, it doesn't look that much better than a gauss rifle, if not worse. The speed - 300 meters per second, or roughly 975 feet per second - isn't that impressive. It's actually not that much faster than a paintball gun - some legal-to-own-without-a-permit air rifles (in the US) fall only 25 feet per second short of that. Weapons with similar rates of fire are hitting about 3000 feet per second. Sorry about not having metric/SI numbers, but I'm mostly talking US guns and ratings, because that's what I'm used to.

 

The article you gave says that it's placing the rounds 8.5 millimeters behind each other. I misinterpreted what you said, sorry. While this is impressive, it's not as impressive as one would think - a Vulcan minigun can spit out over 10 000 rounds a minute. Converting between the two is a pain, though. However, 8.5 millimeters is impressive nonetheless - it's just been done before, IIRC.

 

It says nothing about accuracy. This is quite possibly the deciding factor - I don't care if you can shoot a million rounds a second, if you shoot for five minutes and can't hit a target fifty meters away, it won't do you any good. But it seems viable. I think power consumption would be a large concern, though - as well as the fact that spherical rounds have been tried and proven to be inferior to bullet-shaped projectiles. Interesting, though.]

[calbiterol]

I just found the rate of fire statistic, on the promotional video. http://defensereview.com/dad/dread.mov .

 

"the DREAD offers an extraordinary rate of fire - up to 120 000 rounds per minute in controlled bursts"

 

While this is absolutely INSANE, (wicked!!!), the controlled bursts part is key.

[d22k]

unfortunately i couldn't see the vid, quicktime will not work on my computer (looooong story).

 

While what you say is true, you must remember that this weapon in its current state has been developed by an "amateur", ex soldier granted, but they don't design the guns.

So i wouldn't be surprised if some experts were put on the job, it could exceed capabilities.

 

Having said that, i just thought it was cool and that u may have been interested

 

with regards to spherical rounds, your quite correct, however, i would expect the gun to be used as either a support weapon and/or a CQB where accuracy isn't as important, who knows

 

As for the gauss gun, i was assuming they were going to be using electro magnets OR superconductors, which would need a power source to power/cool.